Autobrake systems reduce pilot workload during landing, by automatically applying smooth, even braking pressure to maintain a programmed deceleration value. They also reduce workload in a rejected take-off (RTO) scenario, helping to bring the aircraft to a halt as rapidly as possible.
The system is armed by the pilot, and then works automatically based on flight inputs, notably whether there is weight on the wheels (compression of nose or main landing gear struts). Most systems automatically disengage if the pilot makes a manual braking input.
In normal use, lower autobrake settings are used to prolong tyre, brake and runway surface life. Additionally, since the system measures total deceleration, use of other techniques to slow the aircraft, such as reverse thrust or spoilers, will contribute to deceleration and hence reduce the required brake application.
By way of example, the system in the 777 behaves as follows:
- Activation occurs with the nose gear strut compressed. Depending on setting, the target deceleration is 3, 5, 7 or 11 (max) ft-sec^2. Any pilot braking input will disengage the system. The braking effort can be changed before or during brake application.
- In RTO mode, the system arms when ground speed exceeds 80 knots, and activates if the throttles are moved to idle above that speed, while the aircraft is still on the ground (nose gear strut compressed). Once airborne, the system automatically disarms. RTO braking effort corresponds to the maximum autobrake setting, 11 ft-sec^2 deceleration.